Spinal analgesic actions of the new endogenous opioid peptides endornorphin-1 and -2

Two highly-selective μ-opioid receptor agonists, endomorphin-1 and -2, were recently purified from bovine brain and are postulated to be endogenous μ-opioid receptor ligands. We sought to determine the effects of these ligands at the spinal level in mice. Endomorphin-1 and -2 produced short acting, naloxone-sensitive antinociception in the tail flick test and inhibited the behavior elicited by intrathecally injected substance P. Both endomorphin-1 and -2 were anti-allodynic in the dynorphin-induced allodynia model. Although acute tolerance against both endomorphins developed rapidly, endomorphin-1 required a longer pretreatment time before tolerance was observed. We conclude that the endomorphins are potent spinal antinociceptive and anti-allodynic agents and that they or related compounds may prove therapeutically useful as spinal analgesics.